1. Field of the Invention
The invention relates to a powder for a magnetic core using a soft magnetic powder, a method for manufacturing the powder for a magnetic core, and a dust core, and more particularly to a powder for a magnetic core obtained by subjecting the surface of a soft magnetic powder to siliconizing.
2. Description of the Related Art
A dust core (powder molded body) can be manufactured by compacting and molding a powder for a magnetic core. An important feature of the dust core is that magnetic properties corresponding to applications are ensured, while ensuring electric insulation between soft magnetic particles constituting the powder for a magnetic core. Accordingly, a large number of dust cores have been researched and developed.
For example, when an iron-based soft magnetic powder (iron powder) of pure iron is used as a soft magnetic powder, a dust core with the highest magnetic flux density can be obtained. This is because the pure iron includes no impurities and, therefore, the iron powder is soft and a dust core of a high density can be easily compression molded from the iron powder.
However, because pure iron has a low specific resistance, when a soft magnetic powder of pure iron is compression molded, an eddy current loss of the dust core increases. A method for manufacturing a powder for a magnetic core by adding elemental silicon or elemental aluminum to the pure iron with the object of increasing the resistance inside the iron powder is one of the methods for reducing the eddy current. However, when these elements are added to pure iron, the hardness of iron increases, thereby increasing hardness of the iron powder itself. As a result, the density of the dust core is difficult to increase.
Accordingly, the surface of pure iron powder is sometimes subjected to phosphating, or coated with a resin such as an epoxy resin or a silicone resin. For example, a phosphate coating film formed on the surface of iron powder by phosphating has a small thickness. Therefore, a high-density dust core can be molded, without losing the properties of the pure iron. However, the dust core obtained by compression molding is sometimes annealed to remove strains introduced during compaction molding, and when the annealing temperature exceeds 500° C., the phosphate diffuses in the iron, thereby making it impossible to increase further the annealing temperature. As a result, strains present in the dust core cannot be sufficiently released and hysteresis loss of the dust core can increase.
In the case of coating with a silicone resin, the silicone resin is more stable at a high temperature than phosphates and has higher heat resistance. However, when a pure iron powder is coated with a silicone resin, the silicone resin film is difficult to preserve during compression molding. Furthermore, because the annealing temperature is raised to about 600° C., a thick silicone resin film has to be coated. As a result, the density of the iron powder for a dust core decreases with the increase in the film density, and the magnetic flux density of the dust core decreases.
A large number of attempts have been made to siliconize (silicide) the iron surface with the object of enriching it with silicon. The siliconizing treatment is typically performed by chemical vapor deposition (CVD) using a silicon tetrachloride gas as a treatment gas.
Accordingly, the possibility of improving magnetic properties by siliconizing has attracted attention and, for example, a method for manufacturing a powder for a magnetic core by performing siliconizing by CVD of a soft magnetic powder under a heating atmosphere of a silicon tetrachloride gas and argon has been suggested (for example, Japanese Patent Application Publication No. 11-87123 (JP-A-11-87123)). With this manufacturing method, increasing the concentration of elemental silicon in the surface of a soft magnetic powder increases magnetic permeability of a dust core and improves magnetic characteristics in a high-frequency range.
However, when the manufacturing method described in JP-A-11-87123 is employed, because a hazardous silicon tetrachloride gas is used, a special manufacturing apparatus designed with consideration for safety has to be used in the manufacturing method. As a result, the production cost in the manufacture of the powder for a magnetic core increases over that in other methods.